This invention relates to systems for determining if a vehicle seat is occupied by a person and, if so, insuring that the air bag will protect the person during a collision as well as possible.
An air bag comprises an inflatable bag and means for inflating the bag. Air bags are highly desired life saving devices that have performed well in many accidents and saved many lives. However, the bag must be inflated in a very brief time such as {fraction (1/30)} of a second which requires rapid movement of the bag from a stored and compacted state to a fully inflated state. The rapid deployment of the bag involves great force. A deploying bag can injure a person during the early phases of deployment if the person is very close to where the airbag is stored. Another hazardous circumstances is when the occupant is a baby in a rear-facing baby seat. It is also desired to inhibit deployment if there is no person in the seat. Much effort has gone into developing systems for characterizing the occupant and ascertaining the occupant position to meet this need. Proposed systems attempt to ascertain the distance from the inflator to the occupant and systems using sonic and optical ranging for that purpose are well known. These systems are deficient in that they cannot reliably distinguish between an occupant and other things such as road maps, beverage cups, packages and voluminous clothing which cause indications that the occupant is near the inflator. Known prior art systems operate to measure the distance from the inflator to the occupant, presumably because that is the physical variable most easily related to the potential for injury.
Many vehicles include an accelerometer located in the passenger compartment for sensing the deceleration of a crash. These accelerometers are incorporated in sensing and diagnostic modules or xe2x80x9cSDMsxe2x80x9d which are decision making centers for the vehicle occupant protection system. The output of the accelerometer may be integrated by an analog circuit or a microprocessor in the SDM to compute a difference between the velocity the vehicle was traveling before a crash and the velocity of the passenger compartment during the crash. The integral of the accelerometer output may be integrated again to obtain the second integral of the deceleration which is the displacement of a free body from its initial position relative to the vehicle. An occupant not wearing a seat belt is, to a good approximation, a free body. Therefore, this calculation provides the distance an unbelted occupant has moved from his or her initial position at any time during the crash. Vehicles typically include seat belt latched sensors for indicating seat belt usage.
Ultrasonic distance measurement based on measuring a time period beginning when sound is generated by a sound emitter and ending when an echo from a object at the distance to be measured is received by a receiver located at a point near the sound emitter is well known and has been used for many years in such as focusing systems for cameras. Using ultrasonic distance measurement to measure the distance from the back of a vehicle seat to the back of a seat occupant works well at larger distances that provide time for vibrations excited during the sound transmission to subside and leave the receiver responsive to low intensity sound.
Position and angle sensors are in commercial production for sensing the position of a seat on its track and the angle the seat is reclined.
Capacitive proximity sensors have been well known for many years and have many successful applications. In addition to measuring capacitance, the Q of the capacitance may be measured to provide additional information about the nature of the material being detected. Some materials including materials containing water tend to significantly reduce the Q of the sensed capacitance.
Ignoring the self inductance of the lead wires, a capacitor is conventionally viewed as a pure capacitor having capacitive reactance       X    C    =      1          2      ·      π      ·              f        (        frequency        )            ·              C        (        capacitance        )            
in series with an energy dissipating resistance RC and the combination has an impedance
Z={square root over (R22+XC2)}
The Power Factor (PF) is defined as the ratio of the effective series resistance RC to the impedance Z and is usually expressed as a percentage.
The Dissipation Factor (DF) is the ratio of the effective series resistance RC to capacitive reactance XC and is usually expressed as a percentage. The DF and PF are essentially equal when the PF is 10 percent or less.
The Quality Factor (Q) is a figure of merit and is the reciprocal of the dissipation factor DF, Q=XC/RC.
Circuits for measuring capacitance and the Q of a capacitor are well known and are incorporated in many commercially available measuring instruments.
The concept of the impedance of a capacitor leads to measuring the capacitance of a capacitor by applying an alternating current voltage to the capacitor and measuring the displacement current through the capacitor. The impedance Z is equal to the applied voltage divided by the current, Z=V(voltage)/I(displacement current). The current leads the voltage by a phase angle (phi).
If the Q of the capacitor is large, RC can be ignored, Z and XC are approximately equal, and the capacitance is obtained directly from the displacement current and the frequency and voltage of the applied alternating current       C    (    capacitance    )    =            I      (              displacement        ⁢                  xe2x80x83                ⁢        current            )              2      ·      π      ·              f        (        frequency        )            ·              V        ⁡                  (          voltage          )                    
For smaller Q or greater precision, the capacitive reactance XC, the resistance RC, and the capacitance are calculated from:
XC=Zxc2x7sin(phi) RC=Zxc2x7cos(phi)
  C  =      1          2      ·      π      ·              f        (        frequency        )            ·              X        C            
The page 322 of the book Electrical Instruments and Measurements by Walter Kidwell and published in 1969 by McGraw-Hill, Inc. states that xe2x80x9cCapacitance can be measured in a number of waysxe2x80x9d. It further states xe2x80x9cGenerally, there are two practical ways of measuring capacitance:
xe2x80x9c1. Absolute measurements in terms of other electrical units.xe2x80x9d
xe2x80x9c2. Comparison methods, where the unknown capacitor is compared with a known standard which has been previously calibrated.xe2x80x9d
xe2x80x9cBridge methods are in the latter category, and it is to these methods that we shall confine our discussion on the following pages.xe2x80x9d
The aforementioned book then proceeds to illustrate a Wien Bridgexe2x80x9d, a xe2x80x9cGeneralized capacitance bridgexe2x80x9d, a xe2x80x9cFive terminal bridge networkxe2x80x9d, a simplified method of connecting a three terminal network, a xe2x80x9cSchering bridgexe2x80x9d, a xe2x80x9cshielded Schering bridgexe2x80x9d, and a bridge having a xe2x80x9cWagner groundxe2x80x9d.
All of the capacitance bridges share the common feature of presenting an alternating current signal to the series combination of an unknown capacitor and a first known element(s) of the bridge. Other elements of the bridge with known properties form a second voltage divider producing a signal for comparison with the signal at the junction between the unknown capacitor and the first known element of the bridge. When the bridge is balanced, the amplitudes and phases of currents in all of the elements of the bridge can be calculated relative to the amplitude and phase of the alternating current signal. Therefore, the illustrated capacitance bridges operate by a process that determines the amplitude and phase shift of the current in the capacitor.
The following two paragraphs illustrate by using the examples of the Wien bridge and the Schering bridge cases of capacitance measurement accomplished by applying a signal to a first plate of a capacitor and observing the signal at the other plate of the capacitor.
FIG. 3 illustrates a Wien bridge. It is reproduced from FIGS. 10-15 of the aforementioned book Electrical Instruments and Measurements. Pages 322 through 329 of this book describe methods for measuring capacitance. In FIG. 3 the parallel combination of Cd and Rd represent respectively the lossless and lossy properties of the capacitance to be measured. In FIG. 3 signal generator 212 provides a signal to plate 214 of capacitor Cd. The signal at plate 216 of capacitor Cd is observed and compared with a comparison signal provided by the resistors Ra and Rb by such as the illustrated headphones. The components Rc and Cc are varied to achieve a balance wherein there is no signal across the headphones 218. Uniquely to the Wien bridge, the lossless part of the capacitance can be determined from the frequency of the signal from the signal generator and the values of the resistors in the bridge without knowing the capacitance of the adjustable capacitor.
FIG. 4 illustrates a Schering bridge. It is reproduced from FIGS. 10-19 of the aforementioned book Electrical Instruments and Measurements. C2, illustrated in FIG. 4, is an unknown capacitor which has lossless and lossy properties. In FIG. 4 signal generator 312 provides a signal to plate 314 of capacitor C2. The signal at plate 316 of capacitor C2 is observed and compared with a comparison signal provided by capacitor C1 in series with the parallel combination of capacitor C4 and resistor R4 by such as the illustrated galvanometer 318. The components C4 and R4 are varied to achieve a balance wherein there is no signal across the galvanometer 318.
The features common to the Wien bridge and the Schering bridge are that an alternating current signal generator is connected to the series combination of the unknown capacitor and a known impedance. By balancing the bridge, the current and phase shift in the unknown capacitor are determined from which the parameters of the unknown capacitor are calculated.
It is well known to use a fluid filled bladder placed under a seat cushion to sense a seat occupant by measuring the pressure in the fluid. When the seat is greatly reclined the weight of the occupant applies less force to the bladder. Applying a factor dependent on seat back recline presents the concern that a child sitting upright would be confused with a heavier adult seated against the seat back.
A general object of this invention is to provide an occupant position sensing means and associated decision making for automotive vehicles which also overcomes certain disadvantages of the prior art.
The present invention provides means for categorizing seat occupants and, if the occupant is a person, the distance from the back of the seat to the back of the person is ascertained.
Further, in accordance with this invention, capacitor electrodes and means responsive to the capacitance between the electrodes are provided to categorize and determine the position of an occupant, the system being highly reliable and economical to manufacture.
Further, in accordance with this invention, a first electrode of a capacitor is located in the seat back of a vehicle seat and a second electrode of the capacitor is located in the seat cushion of a vehicle seat.
Further, in accordance with this invention, the position of a person in a seat is determined from the distance from the back of the seat to the back of the person. This is preferable to ascertaining the position of a person by measuring the position of the front of the person because it is unusual for there to be objects likely to affect capacitance between the back of the seat and the back of the person whereas a person is likely to place objects in front of himself or herself which confuse known measuring systems which are based on reflected sound or light.
Further, in accordance with this invention, the distance from the back of the seat to the back of the seat occupant is calculated from the capacitance between a capacitor electrode in the seat cushion and a capacitor electrode in the seat back when the distance from the back of the seat to the back of the occupant is small and can be accurately measured by capacitance sensing and ultrasonic or radar distance sensors may be inoperative.
Further, in accordance with this invention, at intermediate distances from the back of the seat to the back of the seat occupant, two measuring systems are operable. There is a sensor suitable for measuring larger distances. The capacitance based distance sensing system is used to calibrate and validate the distance sensor suitable for measuring large distances. This enables detection of and possible compensation for factors such as a thick outer coat or a pad placed over the seat back which may lead to inaccurate measurement or errors by an ultrasonic distance sensor or a radar distance sensor.
Further, in accordance with this invention, when the back of the seat occupant is located at large distances from the seat back that make distance calculated from capacitance inaccurate, the distance is measured by a means suitable for measuring large distance such as ultrasonic ranging or radar.
Further, in accordance with this invention, a second pair of capacitor plates and second capacitance sensing means enable calculating a second value at a lower height, of the distance from the seat back to the back of the seat occupant. The first and second measurements together define a line in contact with the back of the seat occupant which defines a plane transverse to the travel direction of the vehicle that defines the position of the back of the seat occupant.
Further, in accordance with this invention, the second measurement at a lower height enables additional determinations such as if the lap belt but not the shoulder belt is functioning.
Further, in accordance with this invention, a highly advantageous seat occupant presence and position sensing system is provided because, by ascertaining if a seat is occupied by a normally seated person or is vacant other important conditions such as forward or rearward facing child seats are also identified.
Further, in accordance with this invention, occupants that have moved into positions where airbag deployment would be dangerous are detected. An occupant reaching forward to place or retrieve something on the dash may be dangerously close to the inflator for a short time. By determining that the back of the occupant is located at a position suggesting a deploying airbag might injure the occupant, this invention enables the air bag to be disabled to eliminate the risk of injury. Being voluntarily out of position is to be distinguished from the occupant being close to the airbag door during an accident which is likely to happen if the seat belt is not in use or is being used incorrectly. The movement of the seat occupant during a crash when the seat belt is not fastened is predicted by doubly integrating the vehicle deceleration and adding the integral and a term resulting from the forward velocity of the occupant when the crash begins to accurately compute the movement of the seat occupant from the position before the crash to positions reached during the crash.
Further, in accordance with the invention, a weight sensor may be provided for measuring the weight of the seat occupant. This enables estimating the distance from the back of the occupant to the front of the chest by assuming the occupant is a person having average dimensions for a person of the measured weight.
Further, in accordance with this invention, a cushion based weight sensor may be provided such as the combination of a fluid filled bladder and a pressure sensor responsive to pressure in the fluid, in combination with a seat back recline sensor. The pressure in the fluid being responsive to the downward force applied by the seat occupant. The capacitance sensor of the invention may be used to distinguish between a child sitting upright or leaning forward from a normally seated adult to enable adjusting estimated weight for the amount of seat back recline.
Further, in accordance with this invention, occupants that remain in dangerous positions are detected. For example, a person might lean forward constantly. One reason a person might lean forward constantly is if the person is very short and needs to lean forward to see the road. Another possible reason is the need to see the road near the front of the vehicle during snow or fog. A person stretching forward and upward to get a better view is particularly vulnerable to injury by a rearward deploying air bag.
Further, in accordance with this invention, the occupant presence and position sensing system of the invention measures the distance the occupant is leaning forward from the normal position against the seat back. This determines an occupant position relative to the seat back based upon the assumption that the occupant is leaning forward so that the lower back of the occupant is at the surface of the seat back. The position of the occupant relative to the seat back is combined with the position of the seat cushion and back calculated from the seat track position and the seat back recline angle to determine the position of the occupant""s back relative to the vehicle structure. The distance from the back to the front of the occupant is assumed to be average for a person of the measured weight. The position of the front of the occupant is used to calculate the distance from the occupant to the airbag. This information is used to prevent air bag deployment when the seat occupant is positioned where deployment would be dangerous.
Further, in accordance with this invention, the occupant presence and position sensing system of the invention measures the distance the occupant has moved forward from the normal position against the seat back. A second electrode and a capacitance sensor enable calculating the distance from the back of the occupant to the surface of the seat at a lower height. The two points determine a straight line in contact with the back of the seat occupant. The position of the seat back calculated from the seat track position and the seat back recline angle is combined with the occupant position relative to the seat to determine the position of the occupant relative to the airbag. This information is used to prevent air bag deployment when the seat occupant is positioned where deployment would be dangerous.
Further, in accordance with this invention, the capacitive presence and position sensing system may also measure the Q of the capacitances being sensed to thereby ascertain additional information about the occupant of the seat.
Further, in accordance with this invention, certain seat occupation categories are each represented by distinct ovals in the Qxe2x80x94log C plane thereby providing a simple method for interpreting a measurement of capacitance and Q.
Further, in accordance with this invention, the occupant presence and position sensor of the invention continuously determines the position of the seat occupant relative to the seat. The position of the seat occupant relative to the seat is communicated to a microprocessor which combines the occupant position with the position of the seat determined from the seat track and recline measurements to continuously make available the position of the occupant relative to the structure of the vehicle.
Further, in accordance with this invention, during a collision, an initial position of the seat occupant relative to the structure of the vehicle is combined with the second integral of the acceleration measured by the accelerometer in the SDM to determine the position of an unbelted seat occupant relative to the vehicle interior as the crash progresses. The occupant position is estimated to be the initial position adjusted forward by the amount of the second integral. If only the lap belt is functioning the head and upper torso will move forward according to the second integral but the lower parts will not, the torso moving in a motion resembling rotation about the seat belt.
Further, in accordance with this invention, differences in seat occupant position may be taken at periodic times to ascertain any forward velocity the occupant might have with respect to the seat. During a collision, the velocity so determined times the time since the initial position was determined is added to the second integral of the deceleration to obtain an estimate of the current position of the seat occupant.